Graphene, a single layer of carbon atoms arranged in a hexagonal lattice (see Wassei et al. (2010) and Bonaccorso et al. (2010)), has unique properties of high carrier mobility, high optical transmittance, chemical inertness and flexibility, making it attractive for optoelectronic applications (see Bonaccorso et al. (2010)). In particular, graphene may provide a promising alternative to indium tin oxide (“ITO”) as transparent conductors (“TC”) (see Bonaccorso et al. (2010)). Currently ITO is widely employed as a transparent conductive electrode with a sheet resistance of as low as 10-30 Ω/square and transparency up to 90% at 550 nm wavelength (see Wassei et al. (2010)). However, long-term use of ITO has severe limitations. It is scarce and consequently becomes prohibitively expensive as the massive demand for photovoltaic devices increases. In addition, ITO is brittle and has much reduced transparency at longer wavelengths (λ), which are unfavorable for high-efficiency, broad-band photovoltaic devices on flexible substrates. Graphene has several unique advantages as the TC for photovoltaic due to its high electrical conductivity, remarkable optical transparency in white light, and favorable work function of about 4.42 eV (see Czerw et al. (2002) and Stankovich et al. (2006)). In fact, each sheet of graphene absorbs only about 2.3% of the incident white light due to its unique gapless electronic structure. In particular, the recent work of large-area growth of graphene using chemical vapor deposition (“CVD”) on metal foils has made an important step toward application of graphene for flexible TCs (see Wassei et al. (2010) and Bae et al. (2010)). Further development of graphene-based TC is therefore important to high-performance and low-cost photovoltaic and other optoelectronic devices.
In thin film solar cells, plasmonic structures that promote the collective oscillation of electrons at a resonance frequency at the interface of metal and dielectric material, have been demonstrated effective in enhancing light scattering and hence light absorption (see Atwater et al. (2010)). Progress has been made recently in application of plasmonics in thin film solar cells (see Atwater et al. (2010), especially in Si-based (see Beck et al. (2010)) and organic solar cells (see Kim et al. (2008)). One approach to integrate plasmonics into solar cells is through addition of metal nanoparticles (“NPs”), on which the resonance frequency of the localized surface plasmons is primarily determined by the charge carrier density of metal and can be further tuned by the size, shape, and surrounding dielectric medium of metal NPs (see Stockman (2011)). A variety of metals have been investigated and silver seems to be one of the few favorable ones for visible applications (others include but are not limited to Cu, Au, and Al) due to its plasmonic frequency in the visible range and low Ohmic losses. Enhanced light trapping has yielded increased photocurrents in solar cells with plasmonic structures implemented (see Beck et al. (2009) and Pillai et al. (2007)). Most recently, graphene with plasmonic nanostructures was reported with strong enhancement of efficiency of graphene-based photodetector (see Echtermeyer et al. (2011)). Development of plasmonic graphene is hence of primary importance to its application as advanced TCs for optoelectronic devices.